Problems with mosfets

I try to turn a pc-fan on an off (maybe pwm later).
For this I try to use mosfets but could not get them to run :sweat_smile:.

First try was a P0903bdl

First problem was that it didn't open when I power the gate pin from nodemcu (3.3v).
I assume that is because of the gate threshold of 3v and the difference of 0.3v is to little?

Because of this assumption I applied 12v to the gate pin. That seemed to work, fan started spinning but soon I noticed that I can't stop it even with gate pin to ground.
All wires detached I can measure continuity between source and drain... So I think it's toast?! But why?

Next victim/try was a sm2558n

I hoped that it will work because of the lesser gate threshold (2.5v) but this thing acts strange.
With nothing attached there is no continuity between source and drain. But when I apply 12v to the source it gets conductive no matter what I do with the gate ( ground/not connected).
Is this normal behavior or is that one broken to?

I never worked With mosfets before and I think I miss something important when locking at the datasheets :sweat_smile:

Post photos of the project.

Post a schematic, too.

I really don't know if there's a MOSFET that can be properly controlled with 3.3V. The threshold voltage is where it starts to conduct. You want to "slam" it on (saturate it).

A regular (bipolar) transistor might work better or the MOSFET might need a transistor driving it. (A transistor turns-on with less than 1V but you generally get more voltage drop/loss across a transistor which means it gets hotter and can burn-up easier.)

No, it should turn off when the gate is grounded. You didn't show us your schematic. Does your circuit have a flyback diode? When you disconnect or turn-off an inductive load there is a high-voltage "kickback" that can fry a MOSFET or transistor.

MOSFETs for Arduino –

Simple MOSFET Switching Circuit – How to turn on / turn off N-Channel and P-channel MOSFETs (
Good luck......

"schematic" and picture of the project.
The gate pin is normally directly connected to the arduino, it is now a jumper pin so could switch it easily..

I first thought of using transistors but the one I had on hand couldn't handle the current from the fan. I think driving the mosfet with a transistor is the way to go. Hoped there was a easier way.

Does the fan power supply ground connect to the MOSFET source and Arduino ground.

Your "schematic" does not show the Arduino nor does it show any power supply(s). The MOSFET pins are not labeled. The MOSFET part number is not shown. The more complete and accurate the information that you supply, the better the assistance will be.

For a N channel MOSFET the + side of the fan connects to the + of the fan power supply. The - of the fan connects to the drain of the MOSFET and the Arduino ground. The gate connects to an Arduino pin set to OUTPUT. We don't know the type of the fan but it may need the flyback diode. Include it if not sure, it can't hurt.

The resistor are optional as is the cap. The 180Ω gate resistor limits the current to the gate when the MOSFET is turned on and the 10K resistor holds the MOSFET off while the Arduino pin is Hi-Z
during processor reset.

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Totally wrong. The thumb is the only correct part! :rofl:

You need a logic-level FET that will actually switch on at 3.3 V such as the IRLR7843. The gate threshold voltage is basically immaterial except that it must be far less than the control voltage.

And used in the correct circuit in #6 above.

That particular MOSFET, P0903BDL, requires an absolute minimim of 5.0V to work, in fact I wouldn't called it logic-level unless its rated to switch at 4.5V (logic supplies can vary a bit). It certainly not compatible with 3.3V logic.

This is a standard 5V logic-compatible MOSFET - but not rated for 3.3V (although you may be lucky, depending on device-spread)

You apply 12V to the drain, not the source, you're just seeing the body-diode conduct I think.

Wrong circuit - you have shown a source-follower - this is not how to connect a MOSFET as a switch (you need common-source circuit, load on the drain).

Where did you get this bad circuit from?

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